1. Field of the Invention
The present invention relates to a crash sensor, and more specifically to a crash sensor suited for sensing collision used in the trigger system of an inflatable air bag or the like mounted in a vehicle that provides protection for an occupant of the vehicle.
2. Description of the Prior Art
U.S. Pat. No. 4,329,549 discloses a known crash sensor in the art, which is described by reference to FIGS. 9 through 11. The crash sensor 101 comprises a magnet 102, a sensing mass 103 made of a ferromagnetic material and attractable by the magnet 102, a sleeve 104 made of a paramagnetic material and permitting the sensing mass 103 to move in one direction only, a pair of cantilever elastic strips 105a and 105b that are located facing each other and will make a closed circuit when contacted by the sensing mass 103, and a plastic body 106 having a projection 106A that fits the magnet 102 and housing airtightly the sensing mass 103, sleeve 104 and elastic strips 105a and 105b. A supporting board 108 constituting part of the body 106 fixes the elastic strips 105a and 105b with its fixed ends 105c projecting in the axial direction and is integrated with the body 106 via a seal 107. The seal 109 is provided to prevent air from flowing into the back of the sleeve 104 through the clearance between the sleeve 104 and the body 106. The fixed end 105c is, as shown in FIG. 11, connected to a printed board 110, the lead wire 110a of which is connected to a connecting terminal 111 provided on a casing 112.
The crash sensor 101 is not directly mounted on a car body, but housed, with its periphery being fixed with a fixing material 113 such as resin, in an open firm casing 112, in view of shock resistance (i.e. to prevent the sensor from breaking before sensing a crash) and resistance to environmental conditions (i.e. to prevent the apparatus from rust generation and the like during a long period while being mounted on a car body with no attention paid thereon). Upon being fixed with the fixing material 113, the crash sensor 101 if positioned by permitting the magnet 102 of the crash sensor 101 to be attracted onto the side wall of the casing 112 and placing the crash sensor 101 on projections 112a of the casing. Where the open casing 112 is made of steel plate, the steel plate also acts as a magnetic shield for protecting the magnet 102 from being influenced by the surroundings but it does not positively play a role of controlling the flux of the magnet 102.
The crash sensor 101 having the above construction operates as follows. When a crash occurs at the left side in the axial direction in FIG. 9, the sensing mass 103 overcomes the attracting force exerted by the magnet 102 and moves in the direction of the arrow to contact with the cantilever elastic strips 105a and 105b. The cantilever elastic strips 105a and 105b being applied by an electric voltage with a cell 114 or the like, the contacting of the sensing mass 103 with the strips 105a and 105b completes an electrical circuit to permit electric current to flow and activate an electric detonator 115 or the like of the air bag system. The magnitude of a crash to be sensed is set by the 3 elements of the magnetic biasing force (pulling-back force) of the magnet applied on the sensing mass 103, the travelling distance L of the sensing mass 103 from its original position to the strips 105a and 105b and the clearance .epsilon. between the sensing mass 103 and the sleeve 104.
The above-described crash sensor 101 is so constructed as to exert a required biasing force on the sensing mass 103 only by the magnet 102 and hence the travelling distance L is restricted by the shape and degree magnetized of the magnet 102 and the size of the sensing mass 103. In such a case, since the magnetic flux of the magnet 102 is only partly incident on the sensing mass, most of it scattering away, it is impossible to sufficiently control the attenuation with distance of the magnetic force by the shape and the like of the magnet 102. The travelling distance L of the sensing mass 103 must therefore be shortened, thereby causing the manufacturing error in the size of parts such as the body 106 to influence the sensitivity relatively largely so that the dispersion in the sensitivity of this crash sensor will become large.
Another problem in this crash sensor 101 lies in the following point. The whole length, L1, of the crash sensor 101 should necessarily become large because of its construction comprising a pair of the slant cantilever elastic strips 105a and 105b with their fixed ends projecting from the supporting board 108 in the direction of crash, and the printed board 110. Further the pair of the slant cantilever elastic strips 105a and 105b are facing each other, whereby the distance W2 between the elastic strips 105a and 105b must be set comparatively large to maintain sufficient elastic recovery force. Then, the supporting board 108 will become of a large size with the body 106 having its maximum diameter D located on the periphery of the supporting board 108. As a result the entire crash sensor 101 should become trumpet-shaped. Accordingly, compactization of the usual crash sensor 101 has been limited due to restrictions from its outer diameter, length and like factors originating from its construction.